Significance of this? extrinsic compression on the thoracic esophagus by the aortic arch in what appears to be left main stem bronchus.

Odd: I assume you are reading the results of a radiology study (recent imaging procedure). The listed structures all lie in close proximity in the mediastinum, the most posterior of which is the esophagus. Unless you are having difficulty swallowing, a cough, or chest discomfort, I wouldn't worry much about this. You might want to confirm by reviewing the report with your doctor.